Heretofore, thermoplastic coated paperboard cartons having flat end closures of the in-folded and/or out-folded fin sealed types have been used with generally satisfactory results. When the, so-called, in-folded type of end closure is used, several panel thicknesses are operative in the folding operations. The multiple panel thicknesses, when folded and sealed, cause buckling or compression distortion of the inner layers or stretching or tensioning of the outer layers around the inner layers to the extent that the outermost thermoplastic layers may tend to pull apart and crack, thereby destroying the liquid impermeable thermoplastic layer, permitting leaking therethrough.
Three known approaches, each of which include supplemental score lines and or embossments to solve such a cracking problem, are shown in Holmstrom U.S. Pat. No. 4,267,957 and in Lisiecki patent application Ser. Nos. 634,718 and 720,314.